The field of glass-ceramics had its origin in U.S. Pat. No. 2,920,971. As is explained therein, glass-ceramic articles are produced through the controlled crystallization in situ of precursor glass articles. The process of manufacture customarily contemplates three basic elements: first, a glass-forming batch to which a nucleating agent is frequently added is melted; second, that melt is cooled to at least within and commonly below the transformation range thereof and a glass article of a desired geometry simultaneously shaped therefrom; and, third, that glass article is exposed to a predefined heat treatment to cause the generation of crystals in situ. Experience has indicated that often more extensive crystallization can be obtained and the crystals are of a more uniform size when the heat temperature step is performed in two stages. That practice involves initially subjecting the parent glass article to a temperature within or somewhat above the transformation range to cause the development of a myriad of nuclei in the glass, and thereafter raising the temperature to approach or even exceed the softening point of the glass to effect the growth of crystals on those nuclei. (The transformation range has been defined as that temperature at which a molten material is converted into an amorphous mass, that temperature generally being deemed as lying in the vicinity of the annealing point of a glass.)
Glass-ceramic articles are normally highly crystalline, viz., greater than about 50% by volume crystalline. Therefore, the articles are usually mechanically stronger than the precursor glass bodies from which they are produced. For example, annealed glass articles conventionally exhibit modulus of rupture values between about 5000-10,000 psi. In contrast, glass-ceramic articles customarily demonstrate modulus of rupture levels in the range of about 10,000-20,000 psi. Whereas the latter values represent a substantial improvement, much research has been conducted in efforts to augment the strength thereof.
The major emphasis in such efforts has involved means for developing surface compression layers on the articles. One method for accomplishing that goal has comprehended applying or forming in situ a surface layer of different chemical or crystalline composition; e.g., through the application of a glaze having a coefficient of thermal expansion lower than that of the glass-ceramic or by heat treating in such a manner to obtain a surface layer of crystals different from the crystal phase in the interior of the glass-ceramic and having a lower coefficient of thermal expansion. A second procedure has utilized a chemical strengthening technique via an ion exchange reaction. The development of a surface compression layer is, indeed, effective in increasing the mechanical strength of glass-ceramic articles, but such a layer is also accompanied with certain practical disadvantages.
First, as can be readily appreciated from the above description, the formation of a surface compression layer involves a further process which obviously adds to the cost of the product. Second and more critically, however, compression strengthening does not heighten the toughness of a glass-ceramic. The property of toughness is extremely important in imparting resistance to catastrophic failure when damage results from impacts received. Hence, where the intrinsic toughness of a body is low, tensile stress concurrently developed in the interior of the body to compensate for surface compression can lead to the body sharply fragmenting into many small pieces upon receiving impacts sufficient to cause fracture. That phenomenon is especially unwanted in consumer products where it is most desirable that any breakage be of a "gentle" nature with but a few large pieces resulting therefrom.
U.S. Pat. No. 4,467,039 is directed to the production of one composition system of glass-ceramic articles which exhibit much improved toughness, coupled with modulus of rupture values in excess of 20,000 psi. Those articles contained potassium fluorrichterite as the predominant crystal phase with the sometime presence of canasite as a secondary phase. Where the material is designed for use in tableware applications, the occurrence of potassium fluorrichterite as the sole crystal phase is preferred. The general composition ranges consist essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 50-70 Na.sub.2 O 2-9 CaO 4-15 K.sub.2 O 2-12 MgO 8-25 Li.sub.2 O 0-3 F 3-8 Al.sub.2 O.sub.3 0-7 ______________________________________
with the preferred compositions utilizing CaF.sub.2 as a nucleating agent and consisting essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 57-68 Na.sub.2 O 2.5-5 Al.sub.2 O.sub.3 0-4 K.sub.2 O 3-7 CaO 0-3 MgO 14-18 CaF.sub.2 7-12 ______________________________________
(Because it was not known with which cation(s) the fluoride was combined, it was reported as CaF.sub.2, the batch constituent through which the fluoride was incorporated into the batch.)
As has been explained in the above general description of the production of glass-ceramic articles, the generation of crystals in situ involves heating to temperatures above the transformation range and, frequently, above the softening point of the parent glass. It will be appreciated that raising the temperature of a glass above its transformation range subjects it to thermal deformation and slumping. Such distortion is self-evidently unwanted since formers or other types of support are required to maintain the desired contour and shape of a glass article. Accordingly, to minimize thermal distortion during the crystallization of a glass body to a glass-ceramic, the temperature is controlled in an effort to grow crystals at a rate sufficient to provide support for the body as the temperature is necessarily raised to achieve maximum crystallization. Yet another factor which must be reckoned with is the quantity and identity of the residual glassy matrix of a glass-ceramic article. Thus, whereas glass-ceramics are frequently very highly crystalline, in some instances over 90% by volume crystalline, a minor amount of glass customarily remains. As can be appreciated, the composition of this residual glass will most commonly be quite different from that of the parent glass body since the components comprising the crystals will have been removed therefrom. Consequently, glass compositions manifesting the least thermal distortion during crystallization to a glass-ceramic will exhibit the following three features:
(1) they crystallize very rapidly upon heat treatment;
(2) the final product is highly crystalline; and
(3) the residual glass is highly refractory.
U.S. Pat. No. 4,467,039 explains that the maximum reduction in glass fluidity during crystallization of the precursor glass to a glass-ceramic, with consequent improved resistance to thermal sagging, is achieved in articles wherein potassium fluorrichterite constitutes the sole crystal phase. But, as was also discussed therein, glass having the stoichiometric composition of potassium fluorrichterite fragments into small pieces as it is being crystallized through heat treatment. That behavior was conjectured to be the result of the glass crystallizing extremely rapidly at temperatures where the glass is at a high viscosity, a condition which prevails where there is very little residual glass. Hence, the above-tabulated ranges of components delineating the preferred compositions illustrate an effort to provide a residual glass phase and to carefully control the constituents of that glass.
Consequently, as is explained therein, a high SiO.sub.2 content was utilized to minimize fluidity of the glass. However, there is an explicit warning against the use of such a high level of SiO.sub.2 that cristobalite crystals are generated during the crystallization of the parent glass body. Minor additions of Al.sub.2 O.sub.3 and/or BaO were suggested to counteract that phenomenon. Furthermore, a careful balance of MgO and CaO contents was demanded to control glass fluidity, as was the relationship of K.sub.2 O and Na.sub.2 O concentrations.
Whereas the glass-ceramic articles of U.S. Pat. No. 4,467,039 do, indeed, demonstrate high toughness and mechanical strength and relatively good resistance to thermal deformation during the crystallization heat treatment, research has continued to improve upon the latter quality.
Accordingly, the primary objective of the instant invention was to devise glass-ceramic compositions displaying high toughness and mechanical strength which exhibit virtually no thermal deformation, as measured through a sag test, during crystallization of the precursor glass.